The present invention can best be understood in the context of its contribution to conventional FCC processes. Accordingly, a brief discussion of conventional cracking processes and catalysts follows.
Conversion of heavy petroleum fractions to lighter products by catalytic cracking is well known in the refining industry. Fluidized Catalytic Cracking (FCC) is particularly advantageous for that purpose. The heavy feed contacts hot regenerated catalyst and is cracked to lighter products. Carbonaceous deposits form on the catalyst, thereby deactivating it. The deactivated (spent) catalyst is separated from cracked products, stripped of strippable hydrocarbons and conducted to a regenerator, where coke is burned off the catalyst with air, thereby regenerating the catalyst. The regenerated catalyst is then recycled to the reactor. The reactor-regenerator assembly are usually maintained in heat balance. Heat generated by burning the coke in the regenerator provides sufficient thermal energy for catalytic cracking in the reactor. Control of reactor conversion is usually achieved by controlling the flow of hot regenerated catalyst to the reactor to maintain the desired reactor temperature.
In most modern FCC units, the hot regenerated catalyst is added to the feed at the base of a riser reactor. The fluidization of the solid catalyst particles may be promoted with a lift gas. Mixing and atomization of the feedstock may be promoted with steam, equal to 1-5 wt % of the hydrocarbon feed. Hot catalyst (650.degree. C..sup.+) from the regenerator is mixed with preheated (150.degree.-375.degree. C.) charge stock. The catalyst vaporizes and superheats the feed to the desired cracking temperature usually 450.degree.-600.degree. C. During the upward passage of the catalyst and feed, the feed is cracked, and coke deposits on the catalyst. The coked catalyst and the cracked products exit the riser and enter a solid-gas separation system, e.g., a series of cyclones, at the top of the reactor vessel. The cracked products pass to product separation. Typically, the cracked hydrocarbon products are fractionated into a series of products, including gas, gasoline, light gas oil, and heavy cycle gas oil. Some heavy cycle gas oil may be recycled to the reactor. The bottoms product, a "slurry oil", is conventionally allowed to settle. The catalyst rich solids portion of the settled product may be recycled to the reactor. The clarified slurry oil is a heavy product.
The "reactor vessel" into which the riser discharges primarily separates catalyst from cracked products and unreacted hydrocarbons, and permits catalyst stripping.
Older FCC units use some or all dense bed cracking. Down flow operation is also possible, in which case catalyst and oil are added to the top of a vertical tube, or "downer," with cracked products removed from the bottom of the downer. Moving bed analogs of the FCC process, such as Thermofor Catalytic Cracking (TCC) are also known.
Further details of FCC processes can be found in: U.S. Pat. Nos. 3,152,065 (Sharp et al); 3,261,776 (Banman et al); 3,654,140 (Griffel et al); 3,812,029 (Snyder); 4,093,537, 4,118,337, 4,118,338, 4,218,306 (Gross et al); 4,444,722 (Owen); 4,459,203 (Beech et al); 4,639,308 (Lee); 4,675,099, 4,681,743 (Skraba) as well as in Venuto et al, Fluid Catalytic Cracking With Zeolite Catalysts, Marcel Dekker, Inc. (1979). The entire contents of these patents and publication are incorporated herein by reference.
Conventional FCC catalysts usually contain finely divided acidic zeolites comprising, e.g., faujasites such as Rare Earth Y (REY), Dealuminized Y (DAY), Ultrastable Y (USY), Rare Earth Containing Ultrastable Y (RE-USY), and Ultrahydrophobic Y (UHP-Y).
Typically, FCC catalysts are fine particles having particle diameters ranging from about 20 to 150 microns and an average diameter around 60-80 microns.
Catalyst for use in moving bed catalytic cracking units (e.g. TCC units) can be in the form of spheres, pills, beads, or extrudates, and can have a diameter ranging from 1 to 6 mm.
Although many advances have been made in both the catalytic cracking process, and in catalyst for use in the process, some problem areas remain.
The catalytic cracking process is excellent for converting heavy hydrocarbons to lighter hydrocarbons. Although this conversion is the whole reason for performing catalytic cracking, the boiling range of the cracked product is frequently not optimum for maximum profitability. Usually the gasoline and fuel oil boiling range fractions are the most valuable materials. Light olefins (C.sub.2 -C.sub.10 olefins) are highly valuable only if a refiner has a way to convert these olefins into gasoline boiling range materials via e.g. alkylation, or if these light olefins can be used for their petrochemical value. Light paraffins, C.sub.10.sup.- materials, are generally not as valuable because of their relatively low octane. The very light paraffins, particularly propane, usually are not as valuable as gasoline. There are ever more stringent limitations on the allowable vapor pressure of gasoline, such that refiners can not blend as much light material into the gasoline as they would like to. Accordingly, there is great interest in converting "top of the barrel" components, or light hydrocarbons in the C.sub.10.sup.- boiling range, into heavier products.
There is also a growing need in refineries to convert more of the "bottom of the barrel" or resid fractions into lighter components via catalytic cracking. Many FCC units today add 5-15 wt % resid, or non-distillable feed, to the catalytic cracking unit. Such heavy materials in the past were never considered as suitable feeds for catalytic cracking units, because of their high levels of Conradson Carbon, sodium, and dehydrogenation metals such as nickel and vanadium. The market for resids (bunker fuel oil, road asphalt) is so limited that refiners have turned to FCC as one way to upgrade the value of the resid fraction.
The most limiting factor in catalytic cracking of resids in conventional FCC units appears to be metals deposition on the catalyst. The nickel and vanadium in the resid deposit almost stoichiometrically on the FCC circulating catalyst inventory, leading to production of excessive amounts of "dry gas" during catalytic cracking. This problem can be ameliorated to some extent by adding metal passivators, such as antimony and/or tin, to passivate the nickel and vanadium components deposited on the catalyst due to processing of resid feed. Usually refiners are also forced to resort to very high levels of catalyst withdrawal and replacement, to maintain the metals levels on the catalyst at a tolerable level, and to maintain catalyst activity. This represents a large daily expense (for make-up catalyst) and presents a disposal problem because the spent catalyst has so much heavy metal on it.
Attempts have been made to modify catalytic cracking catalyst to accomodate heavy feeds. It is known that commercially available FCC catalysts with a high surface area, and an alumina rich matrix, are more resistant to deactivation from metals contamination than other FCC catalysts (Speronello, B. K. and Reagan, W. J., Oil and Gas Journal, Jan. 30, 1984, page 139). See also "Method Predicts Activity of Vanadium-Contaminated FCC Catalyst", E. L. Leuenberger, Oil and Gas Journal, July 15, 1985, page 125.
Another approach to metals passivation is disclosed in U.S. Pat. No. 4,372,841, incorporated herein by reference. Adding a hydrogen donor material to the reaction zone and passing catalyst through a reduction zone at high temperature at least partially passivates the catalyst.
Vanadium, when deposited on a catalyst, is fairly mobile and can migrate to zeolite sites, attack the zeolite and destroy it. This phenomenon was discussed in "Metals Resistant FCC Catalyst Gets Field Test," Jars, Dalen, Oil and Gas Journal, Sept. 20, 1982, Page 135.
Although catalyst manufactures are working on catalysts which apparently can tolerate fairly high levels of metals, and thus permit conversion of more of the "bottom of the barrel" into light products, they have largely ignored the economically related problem of converting light materials, produced during cracking, into more valuable, heavier components.
We have discovered an elutriable, multi component cracking catalyst mixture, and a catalytic cracking process using this catalyst mixture, which is metals tolerant and significantly changes the product distribution from catalytic cracking. We have discovered a way to efficiently convert, in a catalytic cracking unit, the "bottom of the barrel" into more valuable products, and in a preferred embodiment also convert the relatively low value "top of the barrel" materials (incidentally produced during cracking) into more valuable products boiling in the gasoline range. Our elutriable catalyst mixture also allows a measure of control of the amount of "top of the barrel" conversion that occurs.